1. Field of Invention
The present invention relates to a method of fabricating insulating films for application with thin film transistors (TFTs) and metal insulator oxide (MOS) transistors.
2. Description of Related Art
Forming an insulating film, such as SiO2, is a significant step in the manufacture of transistors, such as silicon MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Formation of SiO2 film on silicon is performed at temperatures that are usually higher than 1000xc2x0 C. and in the presence of chemical species that oxidize the silicon. This process is known as thermal oxidation. The thermal oxidation process has undesirable side-effects, such as redistribution of the dopant profiles in the semiconductors, since significant diffusion of dopants occurs at the high temperatures that are used in this process.
Thin film transistor (TFT) devices, which have a basic structure that is similar to that of a typical MOSFET, have been used for display applications, such as liquid crystal displays (LCD) and organic electroluminescence displays (OELD). Such devices require an SiO2 layer to be formed at a temperature that is below 430xc2x0 C., since these displays use an optically transparent substrate, such as glass, which cannot withstand higher temperatures. For such TFT applications, currently deposited Sio2 films are used which are of inferior quality, and which also form an inferior interface with silicon compared to SiO2 that is produced by oxidation of silicon; thereby adversely affecting the TFT performance. Thus, it is required that the oxidation process be performed at temperatures that are as low as possible.
Recently, T. Ueno et al. JJAP 39, pp. L327 (2000) (reference 1) and T. Ohmi et al. Pro. of IDW (1999), pp. 159 (reference 2) reported that applying microwave field to a gas mixture of noble gas and oxygen generates plasma containing atomic oxygen and oxygen radicals, which easily oxidize silicon to form an SiO2 film even at temperatures that are lower than 500xc2x0 C.
The low temperature processes mentioned in the above references involve generation of plasma. Generally, plasma processes are performed at pressures of 1 torr (133 Pa) or less. Additionally, the process-chamber needs to be evacuated to an even lower pressure (base pressure) before the process gases are introduced into the chamber. Thus, these plasma processes require the use of expensive vacuum tools. The maintenance of the vacuum tools further adds to the cost. Vacuum tools also occupy expensive clean room space. Additionally, the microwave plasma processes used above are also limited in their applications. The microwave processes are suitable for semiconductor processing where the maximum size for the substrate is 300 mm in diameter. For the case of TFTs, the substrate size is much larger and approaches 1000 mmxc3x971000 mm in the latest generation equipment. For such large substrate, the microwave plasma process is not suitable.
An object of the present invention is to provide an inexpensive method for fabricating high quality insulating films at low temperatures. Further, it is an object of this invention that such insulating films be fabricated over large substrate, such as those used in TFT fabrication.
In accordance with the invention, high quality SiO2 films are fabricated by plasma excitation of noble gases along with oxygen at the pressures substantially close to atmospheric-pressure (about 100 kPa). This process completely eliminates the need of using vacuum tools, making the equipment and the process very inexpensive compared to equipment and processes used for making similar insulating films in TFT and semiconductor industries. Additionally, radio frequency (RF) power in the MHz range can be used in this process, which makes it possible to apply the process to large substrate.
Additionally, the same process can be used to form silicon nitride by using the nitriding species (such as NH3, N2, etc.) along with noble gases, and creating plasma at pressures that are substantially close to atmospheric pressure.
The method of creating RF plasma at pressures that are substantially close to atmospheric-pressure is advantageous from cost and simplicity perspectives. As a matter of course, even if the process pressure is reduced to be as low as 1 kPa, the processes can be performed with inexpensive vacuum tools.